Vibrating Dams. 377 
light object, as a piece of paper held in the hand. 'The method 
described on p. 365 might perhaps be available. 
2. A minute description of the dam is important. Its precise 
length, breadth, and height; material; form of construction; 
straight or curved ; amount of embankment on the upper side, 
perpendicular or not on the lower side. 
3. Peculiarities in the descending sheet of water, which indi- 
cate the vibrating segments. It seems improbable that a long 
dam should vibrate in one segment. The depth of water on the 
dam should be frequently measured, both during the occurrence 
and cessation of the phenomenon. It should be preserved in a 
register, stating for the same dates whether the vibrations were 
perceived and to what degree. 
4. The direction and force of the wind should be noted when- 
ever the vibrations are suddenly interrupted or resumed. It is 
also important to know what is the greatest distance at which 
the vibrations are ever felt? Are they perceived on all sides of 
the building ; and upon what foundation does the building rest, 
rock, sand, clay or gravel. 
5. It is desirable to get direct evidence of the vibrations of the 
dam. If several rods of an inch diameter were inserted vertically 
into the top of the dam, it is not improbable that by taking care- 
ful range with fixed objects the vibrations might be sensible.- 
This method might detect not only the existence of vibrations, 
but the place of the nodes. In general, we might expect the mid- 
dle of a vibrating segment where the current is swiftest, which 
is usually towards the middle of the dam. If any one should try 
this experiment expecting to see an oscillation of the rods through 
several inches, he would probably be disappointed. I should not 
however despair of being able to render the motion sensible. 
The rods should obviously be sufficiently rigid not to have a vi- 
bratory motion of their own independent of the dam. 
In the speculations in which I have here indulged, I have been 
guided entirely by my own reflections, except so far as the gen- 
eral principles of musical vibrations are concerned. It is not im- 
probable therefore that this article may contain various statements 
which I shall hereafter see occasion to correct. If however it 
should be the means of exciting the curiosity of scientific men, 
my main object will be accomplished. 
Vol. xiv, No. 2.—July-Sept. 1843. 48 
